1. Field of the Invention
The present invention relates to a resist material suitably used for an intermediate layer of a three-layer resist system used when a semiconductor substrate is processed, and to a method of forming a resist pattern using such an intermediate layer material.
2. Description of the Prior Art
Semiconductor devices such as semiconductor integrated circuits are increasingly being integrated at high density. With this tendency, fine-line patterning techniques characterized by high resolution and good linewidth control are required. Recent lithography techniques used for such patterning are versatile and include reduction projection printing using near ultraviolet exposure, electron beam direct writing, and X-ray exposure.
In the manufacturing process of semiconductor elements, topographycal structures frequently generate on a substrate, and predetermined processing must be performed on such a nonplanar surface including a step. However, a resist layer applied to a nonplanar surface brings about a variable thickness. As a result, irrespective of the exposure method adopted, a resist pattern obtained upon development is made non-uniform across the step.
Along with further reduction of a resist pattern size, the influence of a substrate on exposure radiation becomes notable and poor linewidth control is brought about. For example, in the case of optical exposure, a standing wave is produced by reflected light from a substrate. In the case of electron beam exposure, a proximity effect due to reflected electrons occurs. With such phenomena, the resolution and linewidth control of resist patterns are limited.
A so-called three-layer resist system is known as a process to resolve the above problems (see U.S. Pat. No. 4,244,799). This system includes coating a bottom resist layer of an organic polymeric material on a substrate with topography. The bottom resist layer planarize the topography. Thereafter, an intermediate layer is formed on the bottom resist, and a top resist layer of a radiation-sensitive polymeric material is formed thereover.
The top resist layer is patterned by conventional exposure and development techniques. Using the top patterning resist layer as a mask, the intermediate layer is etched. Thereafter, using the obtained intermediate layer pattern as a mask, the bottom resist layer is etched.
According to this three-layer resist system, the intermediate and top resist layers are formed on a planar surface of the bottom resist layer. Therefore, the thin top resist layer to be exposed is coated uniformly, and can be patterned with high resolution. In addition, since the top resist layer is spatially separated from the substrate by a relatively thick bottom resist layer, adverse influence of the substrate on exposure radiation can be reduced. Thus, the three-layer resist system performs high-resolution and good linewidth control.
Despite the advantages of the three-layer resist system as described above, the system requires extra processing steps. In particular, since etching of a bottom resist layer is normally performed by reactive ion etching (RIE) using oxygen, the intermediate layer must be made of a material having resistance to O.sub.2 RIE, i.e., inorganic materials such as silicon, silicon oxide or aluminum. Such an inorganic material is normally formed into a film by evaporation, sputtering or chemical vapor deposition (CVD). These techniques need vacuum equipments which are very costly, and are not so efficient.
In view of this disadvantage, a material capable of being spin-coated is used for the intermediate layer as in the top and bottom resist layers. For example, U.S. Pat. No. 4,244,799 discloses the use of spin on glass (SOG) as an intermediate layer material. However, in order to obtain a stable film of SOG by dehydration condensation of silanol groups, the coated film must be baked at a high temperature above about 200.degree. C. When SOG is exposed to such a high temperature, the bottom resist layer is thermally modified and does not allow easy removal when it must later be removed. In addition, if the baking of SOG is insufficient, it may crack upon development of the top resist layer. SOG gellates or hardens over time causing clogging of a spin coating nozzle and preventing storage over a long period of time.
U.S. Pat. Nos. 4,244,799 and 4,004,044 disclose the use of organopolysiloxanes (so-called silicone resins) for intermediate layers. Since organopolysiloxanes are soluble in organic solvents, they can be spin-coated. Usually, they are baked to cure (crosslink) after coating. The baking temperature is relatively low, i.e., about 100.degree. C. and curing does not therefore modify the bottom resist layer. Since organopolysiloxanes have siloxane bonds, they have satisfactory resistance to oxidation and hence required resistance to O.sub.2 RIE. However, as in the case of SOG, conventional organopolysiloxanes change over time, cannot be dissolved in organic solvents after being cured and thereupon cannot be removed. Unless cured, organopolysiloxanes have low thermal softening points and flow due to heat during prebaking or postbaking of a top resist layer, thus preventing formation of a top resist pattern.